1. Field of the Invention
The invention relates to measuring optical properties of paper or board.
2. Description of Related Art
Measuring optical properties of paper or board is a demanding yet important task. Optical properties, such as opacity, color, lightness, brightness, fluorescence etc., are determined by measuring particularly reflectivity but also transmissivity of paper. Measurement of reflectance relates to measuring reflectance factors of a single sheet of paper or a bundle of sheets. Problems occurring during paper measurement result from the partial translucency of paper, which can be eliminated in laboratory measurements by piling up several sheets of the same paper sample, i.e. by forming a stack of paper to be measured. However, this method is not suitable for on-line measurement of a moving paper web. Problems occur particularly in measurements where the same paper property, such as color, is measured by means of on-line and laboratory measurements, which should correspond to one another.
A measurement result corresponding to stack measurement can be obtained by measuring the reflectance of a single paper web or sheet with a suitable backing being arranged behind the paper. The backing can also create an impression of an infinitely thick stack. It is also possible to use two different backings, i.e. a black backing (highly absorptive) and a white backing (highly reflective). When measurement is carried out by means of these two backings, the Kubelka-Munk theory can be utilized to form reflectance factors for a single sheet of paper and an infinitely thick stack of paper.
One manner of determining reflectance factors is to measure the spectrum of the radiation reflected from a black and a white backing. Since nonsimultaneous measurement of different backings would also lead to measurements being carried out on different parts of a moving paper web, the measurements are preferably implemented simultaneously. Measurements are thus performed by means of two spectrometers, one of which measures the radiation reflected from the black backing and the other one measures the radiation reflected from the white backing. Such an arrangement is disclosed in more detail in U.S. Pat. No. 5,793,486, which is incorporated herein by reference. This arrangement enables for example wavelength-specific opacity corrections before color co-ordinate calculation. However, a drawback is the use of two separate spectrometers, which makes the measuring arrangement more complex and increases costs. Furthermore, when two or more detectors are used for measurement, the detectors should be exactly at the same temperature in order to measure in the same manner. The arrangement disclosed in U.S. Pat. No. 5,793,486 is also sensitive to changes in the distance of the paper web.
An object of the present invention is to provide an improved method and an arrangement implementing the method that reduce complexity. This is obtained by a method of measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. Further, the method comprises illuminating one surface of the target to be measured, the illuminated area on the opposite side of the illuminated surface comprising at least two backings with different optical properties, detecting the illuminated surface in the area of at least two different backings by means of at least a two-dimensional detecting surface, forming at least two spectra from the areas of at least two different backings of the illuminated surface on at least a two-dimensional detecting surface, and determining at least one optical property of the target to be measured by means of the spectra.
The invention also relates to a method of measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. The method further comprises measuring optically the transmittance of the target to be measured, illuminating one surface of the target to be measured, the illuminated area on the opposite side of the illuminated surface comprising at least one backing with known optical properties, detecting the illuminated surface in the area of at least one backing by means of at least a two-dimensional detecting surface, forming at least one spectrum from the area of at least one backing of the illuminated surface on at least a two-dimensional detecting surface, and determining at least one optical property of the target to be measured by means of at least one measured spectrum and measurement of transmittance.
The invention also relates to a method of measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. The method further comprises illuminating the target to be measured from one side and forming an image containing spectral information from the opposite side of the target to be measured, illuminating one surface of the target to be measured, the illuminated area on the opposite side of the illuminated surface comprising at least one backing with known optical properties, detecting the illuminated surface in the area of at least one backing on at least a two-dimensional detecting surface, forming at least one image containing spectral information from the area of at least one backing of the illuminated surface on at least a two-dimensional detecting surface, and determining at least one optical property of the target to be measured by means of the image formed in the area of at least one backing, and the spectral information of an image formed by transmittance measurement.
The invention also relates to an arrangement for measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. The arrangement further comprises an optical power source for illuminating one surface of the target to be measured, at least two backings with different optical properties, located in the illuminated area on the opposite side of the illuminated surface, detecting means for detecting the illuminated surface in the area of at least two different backings, and for forming a spectrum from the areas of at least two different backings on at least a two-dimensional detecting surface, and determining means for determining at least one optical property of the target to be measured by means of at least two spectra that were formed.
The invention also relates to an arrangement for measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. The arrangement is further arranged to measure optically the transmittance of the target to be measured, the arrangement comprising an optical power source for illuminating one surface of the target to be measured, at least one backing with known optical properties, located on the opposite side of the illuminated surface in the illuminated area, detecting means for detecting the illuminated surface in the area of at least one backing and for forming a spectrum from the area of at least one backing on at least a two-dimensional detecting surface, and determining means for determining at least one optical property of the target to be measured by means of at least one formed spectrum and transmittance measurement
The invention also relates to an arrangement for measuring optical properties from paper or board that constitutes a target to be measured and comprises two planar surfaces. The arrangement is further arranged to measure optically the transmittance of the target to be measured, the arrangement comprising an optical power source for illuminating one surface of the target to be measured, and detecting means for forming an image containing spectral information from the opposite side of the target to be measured, at least one backing with known optical properties, located on the opposite side of the illuminated surface in the illuminated area, detecting means for detecting the illuminated surface in the area of at least one backing, and for forming at least one image containing spectral information from the area of at least one backing of the illuminated surface on at least a two-dimensional detecting surface of the detecting means, and determining means for determining at least one optical property of the target to be measured by means of an image formed in the area of at least one backing, and an image formed by transmittance measurement.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on measuring radiation that is reflected from at least two different backings, and on determining desired optical properties on the basis of the measurement. Another possibility is to measure radiation reflected from at least one backing and to determine desired optical properties by means of transmittance measurement.
The method and the arrangement according to the invention provide several advantages. The use of one spectrometer instead of two reduces complexity and the costs of the arrangement, and eliminates problems with temperature between the detectors.